README.md

Async.js

Async is a utility module which provides straight-forward, powerful functions
for working with asynchronous JavaScript. Although originally designed for
use with Node.js and installable via npm install async,
it can also be used directly in the browser.

Async provides around 20 functions that include the usual 'functional'
suspects (map, reduce, filter, each…) as well as some common patterns
for asynchronous control flow (parallel, series, waterfall…). All these
functions assume you follow the Node.js convention of providing a single
callback as the last argument of your async function.

Synchronous iteration functions

If you get an error like RangeError: Maximum call stack size exceeded. or other stack overflow issues when using async, you are likely using a synchronous iterator. By synchronous we mean a function that calls its callback on the same tick in the javascript event loop, without doing any I/O or using any timers. Calling many callbacks iteratively will quickly overflow the stack. If you run into this issue, just defer your callback with async.nextTick to start a new call stack on the next tick of the event loop.

This can also arise by accident if you callback early in certain cases:

Async guards against synchronous functions in some, but not all, cases. If you are still running into stack overflows, you can defer as suggested above, or wrap functions with async.ensureAsync Functions that are asynchronous by their nature do not have this problem and don't need the extra callback deferral.

If javascript's event loop is still a bit nebulous, check out this article or this talk for more detailed information about how it works.

Multiple callbacks

Make sure to always return when calling a callback early, otherwise you will cause multiple callbacks and unpredictable behavior in many cases.

async.waterfall([
function (callback) {
getSomething(options, function (err, result) {
if (err) {
callback(newError("failed getting something:"+ err.message));
// we should return here
}
// since we did not return, this callback still will be called and// `processData` will be called twice
callback(result);
});
},
processData
], done)

It is always good practice to return callback(err, result) whenever a callback call is not the last statement of a function.

Binding a context to an iterator

This section is really about bind, not about async. If you are wondering how to
make async execute your iterators in a given context, or are confused as to why
a method of another library isn't working as an iterator, study this example:

// Here is a simple object with an (unnecessarily roundabout) squaring methodvar AsyncSquaringLibrary = {
squareExponent:2,
square:function(number, callback){
var result =Math.pow(number, this.squareExponent);
setTimeout(function(){
callback(null, result);
}, 200);
}
};
async.map([1, 2, 3], AsyncSquaringLibrary.square, function(err, result){
// result is [NaN, NaN, NaN]// This fails because the `this.squareExponent` expression in the square// function is not evaluated in the context of AsyncSquaringLibrary, and is// therefore undefined.
});
async.map([1, 2, 3], AsyncSquaringLibrary.square.bind(AsyncSquaringLibrary), function(err, result){
// result is [1, 4, 9]// With the help of bind we can attach a context to the iterator before// passing it to async. Now the square function will be executed in its// 'home' AsyncSquaringLibrary context and the value of `this.squareExponent`// will be as expected.
});

Download

The source is available for download from
GitHub.
Alternatively, you can install using Node Package Manager (npm):

Utils

Collections

each(arr, iterator, [callback])

Applies the function iterator to each item in arr, in parallel.
The iterator is called with an item from the list, and a callback for when it
has finished. If the iterator passes an error to its callback, the main
callback (for the each function) is immediately called with the error.

Note, that since this function applies iterator to each item in parallel,
there is no guarantee that the iterator functions will complete in order.

callback(err) - Optional A callback which is called when all iterator functions
have finished, or an error occurs.

Examples

// assuming openFiles is an array of file names and saveFile is a function// to save the modified contents of that file:
async.each(openFiles, saveFile, function(err){
// if any of the saves produced an error, err would equal that error
});

forEachOf(obj, iterator, [callback])

Like each, except that it iterates over objects, and passes the key as the second argument to the iterator.

Arguments

obj - An object or array to iterate over.

iterator(item, key, callback) - A function to apply to each item in obj.
The key is the item's key, or index in the case of an array. The iterator is
passed a callback(err) which must be called once it has completed. If no
error has occurred, the callback should be run without arguments or with an
explicit null argument.

callback(err) - Optional A callback which is called when all iterator functions have finished, or an error occurs.

map(arr, iterator, [callback])

Produces a new array of values by mapping each value in arr through
the iterator function. The iterator is called with an item from arr and a
callback for when it has finished processing. Each of these callback takes 2 arguments:
an error, and the transformed item from arr. If iterator passes an error to its
callback, the main callback (for the map function) is immediately called with the error.

Note, that since this function applies the iterator to each item in parallel,
there is no guarantee that the iterator functions will complete in order.
However, the results array will be in the same order as the original arr.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A function to apply to each item in arr.
The iterator is passed a callback(err, transformed) which must be called once
it has completed with an error (which can be null) and a transformed item.

callback(err, results) - Optional A callback which is called when all iterator
functions have finished, or an error occurs. Results is an array of the
transformed items from the arr.

Example

async.map(['file1','file2','file3'], fs.stat, function(err, results){
// results is now an array of stats for each file
});

Related

mapSeries(arr, iterator, [callback])

mapLimit(arr, limit, iterator, [callback])

filter(arr, iterator, [callback])

Alias:select

Returns a new array of all the values in arr which pass an async truth test.
The callback for each iterator call only accepts a single argument of true or
false; it does not accept an error argument first! This is in-line with the
way node libraries work with truth tests like fs.exists. This operation is
performed in parallel, but the results array will be in the same order as the
original.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A truth test to apply to each item in arr.
The iterator is passed a callback(truthValue), which must be called with a
boolean argument once it has completed.

callback(results) - Optional A callback which is called after all the iterator
functions have finished.

reject(arr, iterator, [callback])

reduce(arr, memo, iterator, [callback])

Aliases:inject, foldl

Reduces arr into a single value using an async iterator to return
each successive step. memo is the initial state of the reduction.
This function only operates in series.

For performance reasons, it may make sense to split a call to this function into
a parallel map, and then use the normal Array.prototype.reduce on the results.
This function is for situations where each step in the reduction needs to be async;
if you can get the data before reducing it, then it's probably a good idea to do so.

Arguments

arr - An array to iterate over.

memo - The initial state of the reduction.

iterator(memo, item, callback) - A function applied to each item in the
array to produce the next step in the reduction. The iterator is passed a
callback(err, reduction) which accepts an optional error as its first
argument, and the state of the reduction as the second. If an error is
passed to the callback, the reduction is stopped and the main callback is
immediately called with the error.

callback(err, result) - Optional A callback which is called after all the iterator
functions have finished. Result is the reduced value.

reduceRight(arr, memo, iterator, [callback])

detect(arr, iterator, [callback])

Returns the first value in arr that passes an async truth test. The
iterator is applied in parallel, meaning the first iterator to return true will
fire the detect callback with that result. That means the result might not be
the first item in the original arr (in terms of order) that passes the test.

iterator(item, callback) - A truth test to apply to each item in arr.
The iterator is passed a callback(truthValue) which must be called with a
boolean argument once it has completed. Note: this callback does not take an error as its first argument.

callback(result) - Optional A callback which is called as soon as any iterator returns
true, or after all the iterator functions have finished. Result will be
the first item in the array that passes the truth test (iterator) or the
value undefined if none passed. Note: this callback does not take an error as its first argument.

Example

async.detect(['file1','file2','file3'], fs.exists, function(result){
// result now equals the first file in the list that exists
});

Related

detectSeries(arr, iterator, [callback])

detectLimit(arr, limit, iterator, [callback])

sortBy(arr, iterator, [callback])

Sorts a list by the results of running each arr value through an async iterator.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A function to apply to each item in arr.
The iterator is passed a callback(err, sortValue) which must be called once it
has completed with an error (which can be null) and a value to use as the sort
criteria.

callback(err, results) - Optional A callback which is called after all the iterator
functions have finished, or an error occurs. Results is the items from
the original arr sorted by the values returned by the iterator calls.

some(arr, iterator, [callback])

Alias:any

Returns true if at least one element in the arr satisfies an async test.
The callback for each iterator call only accepts a single argument of true or
false; it does not accept an error argument first! This is in-line with the
way node libraries work with truth tests like fs.exists. Once any iterator
call returns true, the main callback is immediately called.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A truth test to apply to each item in the array
in parallel. The iterator is passed a callback(truthValue)` which must be
called with a boolean argument once it has completed.

callback(result) - Optional A callback which is called as soon as any iterator returns
true, or after all the iterator functions have finished. Result will be
either true or false depending on the values of the async tests.

Note: the callbacks do not take an error as their first argument.Example

async.some(['file1','file2','file3'], fs.exists, function(result){
// if result is true then at least one of the files exists
});

Related

someLimit(arr, limit, iterator, callback)

every(arr, iterator, [callback])

Alias:all

Returns true if every element in arr satisfies an async test.
The callback for each iterator call only accepts a single argument of true or
false; it does not accept an error argument first! This is in-line with the
way node libraries work with truth tests like fs.exists.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A truth test to apply to each item in the array
in parallel. The iterator is passed a callback(truthValue) which must be
called with a boolean argument once it has completed.

callback(result) - Optional A callback which is called after all the iterator
functions have finished. Result will be either true or false depending on
the values of the async tests.

Note: the callbacks do not take an error as their first argument.

Example

async.every(['file1','file2','file3'], fs.exists, function(result){
// if result is true then every file exists
});

Related

everyLimit(arr, limit, iterator, callback)

concat(arr, iterator, [callback])

Applies iterator to each item in arr, concatenating the results. Returns the
concatenated list. The iterators are called in parallel, and the results are
concatenated as they return. There is no guarantee that the results array will
be returned in the original order of arr passed to the iterator function.

Arguments

arr - An array to iterate over.

iterator(item, callback) - A function to apply to each item in arr.
The iterator is passed a callback(err, results) which must be called once it
has completed with an error (which can be null) and an array of results.

callback(err, results) - Optional A callback which is called after all the iterator
functions have finished, or an error occurs. Results is an array containing
the concatenated results of the iterator function.

Example

async.concat(['dir1','dir2','dir3'], fs.readdir, function(err, files){
// files is now a list of filenames that exist in the 3 directories
});

Related

concatSeries(arr, iterator, [callback])

Control Flow

series(tasks, [callback])

Run the functions in the tasks array in series, each one running once the previous
function has completed. If any functions in the series pass an error to its
callback, no more functions are run, and callback is immediately called with the value of the error.
Otherwise, callback receives an array of results when tasks have completed.

The mechanics and order of enumerating the properties is not specified.

So if you rely on the order in which your series of functions are executed, and want
this to work on all platforms, consider using an array.

Arguments

tasks - An array or object containing functions to run, each function is passed
a callback(err, result) it must call on completion with an error err (which can
be null) and an optional result value.

callback(err, results) - An optional callback to run once all the functions
have completed. This function gets a results array (or object) containing all
the result arguments passed to the task callbacks.

parallel(tasks, [callback])

Run the tasks array of functions in parallel, without waiting until the previous
function has completed. If any of the functions pass an error to its
callback, the main callback is immediately called with the value of the error.
Once the tasks have completed, the results are passed to the final callback as an
array.

Note:parallel is about kicking-off I/O tasks in parallel, not about parallel execution of code. If your tasks do not use any timers or perform any I/O, they will actually be executed in series. Any synchronous setup sections for each task will happen one after the other. JavaScript remains single-threaded.

tasks - An array or object containing functions to run. Each function is passed
a callback(err, result) which it must call on completion with an error err
(which can be null) and an optional result value.

callback(err, results) - An optional callback to run once all the functions
have completed. This function gets a results array (or object) containing all
the result arguments passed to the task callbacks.

until(test, fn, callback)

doUntil(fn, test, callback)

Like doWhilst, except the test is inverted. Note the argument ordering differs from until.

during(test, fn, callback)

Like whilst, except the test is an asynchronous function that is passed a callback in the form of function (err, truth). If error is passed to test or fn, the main callback is immediately called with the value of the error.

forever(fn, [errback])

Calls the asynchronous function fn with a callback parameter that allows it to
call itself again, in series, indefinitely.

If an error is passed to the callback then errback is called with the
error, and execution stops, otherwise it will never be called.

async.forever(
function(next) {
// next is suitable for passing to things that need a callback(err [, whatever]);// it will result in this function being called again.
},
function(err) {
// if next is called with a value in its first parameter, it will appear// in here as 'err', and execution will stop.
}
);

waterfall(tasks, [callback])

Runs the tasks array of functions in series, each passing their results to the next in
the array. However, if any of the tasks pass an error to their own callback, the
next function is not executed, and the main callback is immediately called with
the error.

Arguments

tasks - An array of functions to run, each function is passed a
callback(err, result1, result2, ...) it must call on completion. The first
argument is an error (which can be null) and any further arguments will be
passed as arguments in order to the next task.

callback(err, [results]) - An optional callback to run once all the functions
have completed. This will be passed the results of the last task's callback.

compose(fn1, fn2...)

Creates a function which is a composition of the passed asynchronous
functions. Each function consumes the return value of the function that
follows. Composing functions f(), g(), and h() would produce the result of
f(g(h())), only this version uses callbacks to obtain the return values.

Each function is executed with the this binding of the composed function.

applyEach(fns, args..., callback)

Applies the provided arguments to each function in the array, calling
callback after all functions have completed. If you only provide the first
argument, then it will return a function which lets you pass in the
arguments as if it were a single function call.

Arguments

fns - the asynchronous functions to all call with the same arguments

args... - any number of separate arguments to pass to the function

callback - the final argument should be the callback, called when all
functions have completed processing

queue(worker, [concurrency])

Creates a queue object with the specified concurrency. Tasks added to the
queue are processed in parallel (up to the concurrency limit). If all
workers are in progress, the task is queued until one becomes available.
Once a worker completes a task, that task's callback is called.

Arguments

worker(task, callback) - An asynchronous function for processing a queued
task, which must call its callback(err) argument when finished, with an
optional error as an argument. If you want to handle errors from an individual task, pass a callback to q.push().

concurrency - An integer for determining how many worker functions should be
run in parallel. If omitted, the concurrency defaults to 1. If the concurrency is 0, an error is thrown.

Queue objects

The queue object returned by this function has the following properties and
methods:

length() - a function returning the number of items waiting to be processed.

started - a function returning whether or not any items have been pushed and processed by the queue

running() - a function returning the number of items currently being processed.

idle() - a function returning false if there are items waiting or being processed, or true if not.

concurrency - an integer for determining how many worker functions should be
run in parallel. This property can be changed after a queue is created to
alter the concurrency on-the-fly.

push(task, [callback]) - add a new task to the queue. Calls callback once
the worker has finished processing the task. Instead of a single task, a tasks array
can be submitted. The respective callback is used for every task in the list.

unshift(task, [callback]) - add a new task to the front of the queue.

saturated - a callback that is called when the queue length hits the concurrency limit,
and further tasks will be queued.

empty - a callback that is called when the last item from the queue is given to a worker.

drain - a callback that is called when the last item from the queue has returned from the worker.

paused - a boolean for determining whether the queue is in a paused state

pause() - a function that pauses the processing of tasks until resume() is called.

resume() - a function that resumes the processing of queued tasks when the queue is paused.

kill() - a function that removes the drain callback and empties remaining tasks from the queue forcing it to go idle.

While queue passes only one task to one of a group of workers
at a time, cargo passes an array of tasks to a single worker, repeating
when the worker is finished.

Arguments

worker(tasks, callback) - An asynchronous function for processing an array of
queued tasks, which must call its callback(err) argument when finished, with
an optional err argument.

payload - An optional integer for determining how many tasks should be
processed per round; if omitted, the default is unlimited.

Cargo objects

The cargo object returned by this function has the following properties and
methods:

length() - A function returning the number of items waiting to be processed.

payload - An integer for determining how many tasks should be
process per round. This property can be changed after a cargo is created to
alter the payload on-the-fly.

push(task, [callback]) - Adds task to the queue. The callback is called
once the worker has finished processing the task. Instead of a single task, an array of tasks
can be submitted. The respective callback is used for every task in the list.

saturated - A callback that is called when the queue.length() hits the concurrency and further tasks will be queued.

empty - A callback that is called when the last item from the queue is given to a worker.

drain - A callback that is called when the last item from the queue has returned from the worker.

idle(), pause(), resume(), kill() - cargo inherits all of the same methods and event calbacks as queue

auto(tasks, [callback])

Determines the best order for running the functions in tasks, based on their requirements. Each function can optionally depend on other functions being completed first, and each function is run as soon as its requirements are satisfied.

If any of the functions pass an error to their callback, the auto sequence will stop. Further tasks will not execute (so any other functions depending on it will not run), and the main callback is immediately called with the error. Functions also receive an object containing the results of functions which have completed so far.

Note, all functions are called with a results object as a second argument,
so it is unsafe to pass functions in the tasks object which cannot handle the
extra argument.

tasks - An object. Each of its properties is either a function or an array of
requirements, with the function itself the last item in the array. The object's key
of a property serves as the name of the task defined by that property,
i.e. can be used when specifying requirements for other tasks.
The function receives two arguments: (1) a callback(err, result) which must be
called when finished, passing an error (which can be null) and the result of
the function's execution, and (2) a results object, containing the results of
the previously executed functions.

callback(err, results) - An optional callback which is called when all the
tasks have been completed. It receives the err argument if any tasks
pass an error to their callback. Results are always returned; however, if
an error occurs, no further tasks will be performed, and the results
object will only contain partial results.

Example

async.auto({
get_data:function(callback){
console.log('in get_data');
// async code to get some data
callback(null, 'data', 'converted to array');
},
make_folder:function(callback){
console.log('in make_folder');
// async code to create a directory to store a file in// this is run at the same time as getting the data
callback(null, 'folder');
},
write_file: ['get_data', 'make_folder', function(callback, results){
console.log('in write_file', JSON.stringify(results));
// once there is some data and the directory exists,// write the data to a file in the directory
callback(null, 'filename');
}],
email_link: ['write_file', function(callback, results){
console.log('in email_link', JSON.stringify(results));
// once the file is written let's email a link to it...// results.write_file contains the filename returned by write_file.
callback(null, {'file':results.write_file, 'email':'user@example.com'});
}]
}, function(err, results) {
console.log('err = ', err);
console.log('results = ', results);
});

This is a fairly trivial example, but to do this using the basic parallel and
series functions would look like this:

async.parallel([
function(callback){
console.log('in get_data');
// async code to get some data
callback(null, 'data', 'converted to array');
},
function(callback){
console.log('in make_folder');
// async code to create a directory to store a file in// this is run at the same time as getting the data
callback(null, 'folder');
}
],
function(err, results){
async.series([
function(callback){
console.log('in write_file', JSON.stringify(results));
// once there is some data and the directory exists,// write the data to a file in the directory
results.push('filename');
callback(null);
},
function(callback){
console.log('in email_link', JSON.stringify(results));
// once the file is written let's email a link to it...
callback(null, {'file':results.pop(), 'email':'user@example.com'});
}
]);
});

retry([opts = {times: 5, interval: 0}| 5], task, [callback])

Attempts to get a successful response from task no more than times times before
returning an error. If the task is successful, the callback will be passed the result
of the successful task. If all attempts fail, the callback will be passed the error and
result (if any) of the final attempt.

Arguments

opts - Can be either an object with times and interval or a number. times is how many attempts should be made before giving up. interval is how long to wait inbetween attempts. Defaults to {times: 5, interval: 0}

if a number is passed in it sets times only (with interval defaulting to 0).

task(callback, results) - A function which receives two arguments: (1) a callback(err, result)
which must be called when finished, passing err (which can be null) and the result of
the function's execution, and (2) a results object, containing the results of
the previously executed functions (if nested inside another control flow).

callback(err, results) - An optional callback which is called when the
task has succeeded, or after the final failed attempt. It receives the err and result arguments of the last attempt at completing the task.

The retry function can be used as a stand-alone control flow by passing a
callback, as shown below:

async.retry(3, apiMethod, function(err, result) {
// do something with the result
});

async.retry({times:3, interval:200}, apiMethod, function(err, result) {
// do something with the result
});

It can also be embeded within other control flow functions to retry individual methods
that are not as reliable, like this:

iterator(tasks)

Creates an iterator function which calls the next function in the tasks array,
returning a continuation to call the next one after that. It's also possible to
“peek” at the next iterator with iterator.next().

This function is used internally by the async module, but can be useful when
you want to manually control the flow of functions in series.

nextTick(callback), setImmediate(callback)

Calls callback on a later loop around the event loop. In Node.js this just
calls process.nextTick; in the browser it falls back to setImmediate(callback)
if available, otherwise setTimeout(callback, 0), which means other higher priority
events may precede the execution of callback.

This is used internally for browser-compatibility purposes.

Arguments

callback - The function to call on a later loop around the event loop.

Utils

memoize(fn, [hasher])

Caches the results of an async function. When creating a hash to store function
results against, the callback is omitted from the hash and an optional hash
function can be used.

If no hash function is specified, the first argument is used as a hash key, which may work reasonably if it is a string or a data type that converts to a distinct string. Note that objects and arrays will not behave reasonably. Neither will cases where the other arguments are significant. In such cases, specify your own hash function.

The cache of results is exposed as the memo property of the function returned
by memoize.

Arguments

fn - The function to proxy and cache results from.

hasher - An optional function for generating a custom hash for storing
results. It has all the arguments applied to it apart from the callback, and
must be synchronous.

asyncify(func)

Alias:wrapSync

Take a sync function and make it async, passing its return value to a callback. This is useful for plugging sync functions into a waterfall, series, or other async functions. Any arguments passed to the generated function will be passed to the wrapped function (except for the final callback argument). Errors thrown will be passed to the callback.

Example

async.waterfall([
async.apply(fs.readFile, filename, "utf8"),
async.asyncify(JSON.parse),
function (data, next) {
// data is the result of parsing the text.// If there was a parsing error, it would have been caught.
}
], callback)

log(function, arguments)

Logs the result of an async function to the console. Only works in Node.js or
in browsers that support console.log and console.error (such as FF and Chrome).
If multiple arguments are returned from the async function, console.log is
called on each argument in order.

Arguments

function - The function you want to eventually apply all arguments to.

dir(function, arguments)

Logs the result of an async function to the console using console.dir to
display the properties of the resulting object. Only works in Node.js or
in browsers that support console.dir and console.error (such as FF and Chrome).
If multiple arguments are returned from the async function, console.dir is
called on each argument in order.

Arguments

function - The function you want to eventually apply all arguments to.